A major effort to reducing the risk of a nuclear catastrophe is to deploy radiation detectors within a larger security architecture. One important component of radiation detection is neutron detection, since neutron detectors can aid in the detection of fissile material such as plutonium, and are components of systems that perform active interrogation to detect materials such as highly enriched uranium (HEU). The standard method for neutron detection uses 3He gas within a tube and signals read out by a central high voltage wire. The combination of high probability for the neutron/3He interaction, full energy readout from the reaction, and the simple tube geometry makes the method a gold standard of neutron detection.
The limited supply of 3He and the demand on its use for other activities such as medical imaging and low-temperature research has limited the number of neutron detectors that can be deployed, however one may argue that cost and scalability have not played a large role in the design of 3He replacement technologies. This may limit the ability to expand to larger deployments of thermal neutron detection beyond the task of replacing 3He units. The present invention provides cost effective neutron monitoring which can be used at a very large number of trade routes throughout the world, including shipping, air, and rail travel. The goal is to make a large dent in the completion of a global security architecture by increasing the amount of 3He equivalent neutron detectors by ten to one hundred fold at a fraction of the cost of other technologies.
Slow neutrons can be detected with high efficiency by a few special detector materials. 6Li has a cross section of 940 barns for thermal neutrons in the reaction equation 6Li+n→α+3H. 4.786 MeV of kinetic energy is given off by the alpha (α) and triton (3H) as they travel outward back to back from the reaction point. The mean free path for thermal neutron capture in enriched lithium is 229 microns, whereas the distances that the alpha and triton particles travel with the 4.786 MeV of kinetic energy received by the reaction are 23.3 and 135 microns respectively.
U.S. Pat. No. 4,365,159 by Young et al. shows the use of thin sheets or foils of lithium in a gas-filled multi-wire proportional chamber (MWPC). The lithium foil is sufficiently thin so that large particles created by reactions between incident neutrons and lithium nuclei escape the foil with sufficient energy to cause ionization of a detection gas in the chamber. Drift electrons from the ionizations are attracted to an array of wires in the chamber biased at a high voltage, and the resulting signals induced on the wires are detected and processed by electronic circuitry.
U.S. Pat. No. 4,447,727 of Friesenhahn et al. shows a large area neutron proportional counter constructed utilizing a large sealed metal box. The interior walls of the box are coated with 6Li enriched metal. A multicelled proportional counter structure within the internal space defined by the box is fabricated using a hydrogenous plastic.
US patent application publication 2006/0138340 of Ianakev et al. shows detector for detecting neutrons and gamma radiation that includes a cathode that defines an interior surface and an interior volume. A conductive neutron-capturing layer is disposed on the interior surface of the cathode and a plastic housing surrounds the cathode. A plastic lid is attached to the housing and encloses the interior volume of the cathode forming an ionization chamber, into the center of which an anode extends from the plastic lid. A working gas is disposed within the ionization chamber and a high biasing voltage is connected to the cathode. Processing electronics are coupled to the anode and process current pulses which are converted into Gaussian pulses, which are either counted as neutrons or integrated as gammas, in response to whether pulse amplitude crosses a neutron threshold